Effect of Electric Field Uniformity on Breakdown Mechanisms of Dielectric Liquids under Lightning Impulse

The electric field uniformity imposes a significant effect on the breakdown mechanisms of dielectric liquids. In general, breakdown is dominated by streamer initiation under uniform field, while it is dominated by streamer propagation under non-uniform field. This paper presents a study on identifying and quantifying the transition field factor boundary, represented by the field enhancement factor $f$, between the two dominating breakdown mechanisms. The methods for deducing the transition field factor boundary are introduced. The first method from our previous study was developed by measuring and processing both the streamer initiation and breakdown voltages. In this paper, a new method is developed by only processing the breakdown voltage data. Based on the breakdown results reproduced from the literature, the transition field factor boundaries are derived for the mineral oil under both positive and negative lightning impulses. It is found that the negative transition field factor boundary is lower than the positive one. A four-region model is proposed to describe the effect of impulse polarity on breakdown mechanisms of dielectric liquids and to interpret the transition of breakdown mechanisms with respect to the field uniformity.